Expression and sorting of rat plasma kallikrein in POMC-producing AtT-20 cells

Role of loop structures of neuropsin in the activity of serine protease and regulated secretion

Neuropsin involved in neural plasticity in adult mouse brain is a member of the S1 (clan SA) family of serine proteases and forms characteristic surface loops surrounding the substrate-binding site (Kishi, T., Kato, M., Shimizu, T., Kato, K., Matsumoto, K., Yoshida, S., Shiosaka, S., and Hakoshima, T. (1999) J. Biol. Chem. 274, 4220-4224). Little, however, is known about the roles of these loops. Thus, the present study investigated whether surface loop structures of neuropsin were essential for the generation of enzymatic activity and/or secretion of the enzyme via a regulated secretory pathway. The loops include those stabilized by six disulfide bonds or a loop C (Gly(69)-Glu(80)) and an N-glycosylated kallikrein loop (His(91)-Ile(103)) not containing a site linked by a disulfide bond. First, among the six disulfide bonds, only SS1 in loop E (Gly(142)-Leu(155)) and SS6 in loop G (Ser(185)-Gly(197)) were necessary for the catalytic efficiency of neuropsin. Second, disruptions of loop C and the N-linked oligosaccharide chain on the kallikrein loop affected the catalytic efficiency and P2 specificity, respectively. Alternatively, disruptions of loop C and the kallikrein loop enhanced the regulated secretion, whereas there was no one disruption that inhibited the secretion, indicating that there was no critical loop required for the regulated secretion among loops surrounding the substrate-binding site.

Processing of chromaffin granule proteins: a profusion of proteases?

Evidence suggests that proenkephalin and members of the chromogranin/secretogranin family of proteins are prohormone precursors, giving rise to a variety of peptides with biologic activity. However, the specific proteases responsible for cleaving these proteins in vivo have not been fully established. Several candidate proteases have been described, some of which have been shown to cleave these proteins in vitro. Proteolytic processing of the chromogranins may be particularly complex, occurring in specific tissue-dependent patterns. To account for this level of complexity several protease systems may be operative, either alone or in concert, both within the neurosecretory granule and in the extracellular space. Specific proteases which are available within neurosecretory cells or in the local extracellular environment, and which may cleave these prohormones include PC1 and PC2 (recently described members of the Kex2/furin family of endoproteases), as well as kallikrein, acetylcholinesterase, and, more recently, the plasminogen/plasmin protease system. The potential role of these specific proteases in the processing of proenkephalin and the chromogranins is discussed, in particular, in the context of possible processing clues available from recent analysis of cDNA and genomic intron/exon structure.

Proprotein and prohormone convertases of the subtilisin family Recent developments and future perspectives

Limited proteolysis of precursors at specific pairs of basic residues and/or at single basic amino acids is a widespread mechanism by which the cell expresses a repertoire of biologically active proteins and peptides. The cloning and cellular expression of the yeast KEX2 gene product demonstrated that this enzyme belongs to the subtilisin family of serine proteinases, and that it exhibits exquisite selectivity for cleavage post pairs of basic residues in a number of yeast and mammalian precursors. The search for the homologous mammalian convertases led to the identification and molecular cloning of three new members of the family, furin, PCI, and PC2. Whereas furin is almost ubiquitous, PCI and PC2 localize mostly in endocrine and neuroendocrine tissues and cells. Coexpression of each gene product with proproteins demonstrated that each proteinase selectively cleaved these precursors at distinct pairs of LysArg and ArgArg residues. In human and mouse, the genes coding for furin, PCI, and PC2 reside on three different chromosomes. Overexpression of PO and PC2 in Sf9 cells in the baculovirus system demonstrated that these enzymes are not secreted and that they both retained their N-terminal prodomain.